1. The Field of the Invention
The present invention relates generally to methods and circuits for transmitting information in a communications environment. More particularly, embodiments of the present invention relate to methods and circuitry for modulating optical signals in an optical communications system.
2. The Relevant Technology
Semiconductor lasers are frequently used as optical sources for transmitting information in an optical communication network. An optical network is digital, meaning that data is represented by logical 1s and 0s. In an electrical network environment, a high electrical current or voltage can represent a 1, while a low electrical current or voltage corresponds to a 0. To create these “pulses” from a constant source of electricity, a very fast electrical switching type of circuit can be used. In the optical domain, some kind of switching capability must be provided in order to alter the light signal into a stream of high and low optical power levels to represent digital information. This technique is typically referred to as “modulation,” where the light signal is “modulated” in a manner so as to represent digital information.
Modulation methods for lasers generally fall into two categories: (1) direct modulation, where the bias current applied to the laser is itself modulated by an electrical data signal; or (2) indirect (external) modulation, where the bias current to the laser is held constant, and the constant light output from the laser is modulated by the electrical data signal to provide the light-wave output data signal.
Direct modulation is relatively inexpensive to implement because it requires few components. However, it is problematic in several respects. First, direct modulation techniques are susceptible to “chirp,” where the exact wavelength of the light emitted varies throughout each pulse. This happens because of small changes in the laser material's refractive index due to the varying electrical current being applied to it. Chirp is indesirable because a pulse with a broad range of wavelengths is susceptible to dispersion through the transmission optical fiber.
Direct modulation also becomes problematic at bit rates above a few GHz due to the increased dispersion described above, and due to problems with the laser's so-called “relaxation oscillations.” This phenomenon results from the rapid variations in light output that occur just after a laser has been switched on, and they take time to settle down. This causes difficulties at high bit-rates, thereby limiting the effective communication speeds that can be obtained when direct modulation lasers are used. Finally, direct modulation techniques can be problematic because there are implications for the reliability of lasers that are switched rapidly on and off.
In contrast, external modulation addresses some of these and other problems encountered with direct modulation. Again, when using this technique of external modulation, the electrical current applied to the laser is typically kept constant. As a result, the laser provides a constant level of output optical power. Hence, the problems with direct modulation such as excessive chirp (and therefore dispersion), relaxation oscillations, and laser reliability—all relating to turning the laser on and off—are eliminated or reduced.
In general, an external modulator is a device that is placed between the laser and the output fiber. Simply described, the external modulator is designed to let the laser's light pass when a logical 1 is desired, and to block the laser's light from passing through the external modulator when a logical 0 is desired. Generally, signals from externally modulated lasers can travel much farther (in terms of their dispersion performance), and at higher data rates, than their directly modulated counterparts. Thus, this approach is generally a preferred alternative for multi-gigabit applications.
However, the extra performance provided by external modulation does result in some drawbacks. In particular, external modulation generally requires additional technology in the form of additional electronic components, and dictates the use of additional and alternative power requirements. This in turn gives rise to a number of problems, including additional power dispersion, increased cooling requirements, and increased physical space requirements. Such considerations are especially problematic in those environments where physical space is at a premium, such as in electro-optical transceiver modules conforming with standards such as the Small Form-Factor Module Multi-Source Agreement (SFF MSA), the Small Form-Factor Pluggable Module Multi-Source Agreement (SFP MSA) and the 10 Gigabit Small Form Factor Pluggable Module Multi-Source Agreement (XFP MSA) Revision 4.0, which are incorporated herein by reference.
Physical space problems have been at least partially addressed by utilizing so-called integrated electro-absorption types of optical modulators (sometimes referred to as EA modulators). Electro-absorption modulators are made of similar materials to semiconductor lasers, thereby allowing them to be integrated with the lasers they are to be used with. Thus, the modulator and the laser are fabricated as an integrated device on a common chip substrate. In this environment, light is generated in the laser by applying an electric current to pass through a semiconductor material. Thus, the semiconductor laser must be “forward biased” with an appropriate current. The electro-absorption modulator is similar to the laser, but to prevent light passing through it (to give a 0) it needs a “reverse bias” voltage to give it the requisite light-absorbing properties. To represent a 1, no reverse bias voltage is applied to the electro-absorption modulator, which causes the modulator to behave transparently to the laser light.
Thus, in a typical environment having an electro-absorption modulated laser arrangement, there must be a suitable laser driver configuration. For example, in a typical arrangement, a current source is provided to supply a sufficient forward bias current to the semiconductor laser—typically via a sufficient supply voltage (for example, a 5 volt supply). Here, there must be a sufficient voltage “headroom” to provide a sufficient current to drive the laser. In a typical arrangement, this headroom must be at least approximately 3.3 volts. In contrast, the electro-absorption (EA) modulator requires a voltage that can be reverse biased, below ground, to sufficiently gate the optical signal from the laser. To accomplish this, typical prior art solutions provide a driver having a differential output pair stage that operates between ground and a negative power supply. This allows the EA modulator to be driven with a sufficient negative voltage range (reverse bias) so that it can absorb the light emitted by the laser diode.
This need to provide a sufficient forward bias to supply current to the laser, and a sufficient reverse bias voltage to the EA modulator gives rise to a problem. In particular, this arrangement typically requires an overall higher voltage headroom for driving the diode pair of the externally modulated laser (“EML”) by accommodating both the current requirements of the laser and the reverse bias requirements of the EA modulator. For example, a headroom of 7 volts differential or more may be needed. Thus, if an EML environment is desired, a typical transceiver module might need to provide +3.3, +1.8 and +5 volt supply lines, plus an additional −5.2 volt supply so as to achieve the headroom needed.
FIG. 1, for example, illustrates a DC coupled driver bias network that requires a negative power supply. The RF signal applied to the external modulator 104 is an electrical signal that alters the bias applied to the external modulator 104. Thus, the external modulator 104 either blocks or passes the laser light based on the RF signal.
In FIG. 1, the laser 106 is biased to emit light at a constant optical output. The external modulator 104 is DC coupled to both the RF and bias 110. The bias is −5.2 volts, in this example, and is needed to insure that the modulation (RF signal) can cause the external modulator 104 to either block or pass the light emitted by the laser 106. As previously described, the laser light is modulated as the external modulator blocks or passes the laser light.
In the example of FIG. 1, power dissipation is a problem for various reasons. The bias is DC coupled to the external modulator 104 and therefore dissipates power through the resistor 102, which is needed for proper termination of the RF signal. The dissipation of the negative bias, in addition to the dissipation required for the laser 106, requires additional cooling capability. In turn, the additional cooling capability, often provided by inefficient thermoelectric cooler (TEC), requires additional power. This increases the power requirements of the EML and makes it difficult to comply, for example, with the power requirements specified by the SFF, SFP, and XFP standards.
FIG. 2 illustrates similar problems in a conventional bias T network 200. In FIG. 2, the RF 210 is capacitively coupled to the network 200, but the bias 212 of the external modulator 204 still dissipates power through the resistor 202. The inductor 214 is low impedance to a DC source, but is high impedance to the RF signal. Thus, the resistor 202 provides the proper termination for the RF 210. In addition, the inductor 214 can occupy substantial space and is therefore unsuitable for use in transceivers that conform with the SFP or XFP standards, for example. Further, the network 200 still requires a negative power supply for the bias 212 in order to appropriately bias the external modulator 204.
In general, these arrangements are problematic in that extra power requirements give rise to a number of undesirable factors, including cost (for extra power supply devices), added design complexity, increased power dissipation levels, which in age turn requires additional cooling components (and thus additional electrical power). Consequently, the solution is typically unsuitable, especially given the ongoing objective to physically reduce the size of transceiver modules and keep power requirements within certain bounds.
Thus, it would be an advancement in the art to provide an EML that can be adequately biased without requiring the presence of an extra negative voltage, such as a negative 5.2 volt supply, to the transceiver. At the same time, the approach should provide a sufficient voltage headroom to allow for proper operation of the EML diodes.